The rectal administration of pharmacologically active molecules is widely used. This form of administration has the advantage over oral administration in that it is unaffected by the ingestion of foods, nausea or low intestinal absorption. It can be used in patients who are incapable of salivating, who are unconscious or who have to go without food before an operation.
Nevertheless, since the rectum is made up of zones differing in their characteristics, the effects of treatment by rectal administration vary according to the pharmaceutical forms used and their anatomical positioning.
Thus, the positioning of a pharmaceutical dosage unit in the lower part of the rectum enables the active ingredient which it contains to pass directly into the general circulation. Dosage forms positioned in the upper or middle part of the rectum enable the active ingredients which they contain to pass into the portal system. In that case, the active ingredient is in danger of undergoing metabolic degradation in the liver.
Dosage units which provide for specific administration in the lower part of the rectum have already been described in the literature. They have outer surfaces comprising bioadhesive substances which facilitate their retention by the layer of mucus covering that part of the rectum. This retention prevents the pharmaceutical dosage unit from moving towards the upper parts of the rectum.
Suppositories containing fats are unsuitable for this application because they cannot be retained in the lower part of the rectum on account of their composition. Suppositories made of hydrogels, such as polyethylene glycol, which are impregnated with pharmacologically active substances have also been described (Cole et al., Br J. Clin. Pharmac. 1990, 30, 781-786).
Gelatin sheaths containing various active ingredients have also been tested for rectal administration.
These sheaths are positioned in the lower part of the rectum where they deliver the active ingredient which they contain (Brumley et al., The Hospice Journal, Vol. 4(1) 1988; Maloney et al., The American Journal of Hospice Care, July/August 1989, 6(4), 34-35).
These pharmaceutical dosage units allow the active ingredient to pass into the circulation without being metabolized by the liver.
Nevertheless, this type of pharmaceutical dosage unit is variously accepted by patients with dry rectal mucosa and does not meet all the requirements of treatments involving this method of administration.